1. Field of the Invention
The present invention relates to a post hole digger. In particular, the present invention relates to a post hole digger for digging holes of approximately constant diameter.
2. Discussion of Background
Many types of post hole diggers are known. Auger-type diggers are provided with a screw that is advanced into the ground, such as the motorized auger shown by Ovens (U.S. Pat. No. 4,961,471). Alternatively, a boring implement can be secured to an end of a post to be inserted into the ground. The post is rotated and advanced to the desired depth, such as by directing a stream of water in the direction of the piercing end to cause rotation of the post (Charland, et al., U.S. Pat. No. 4,986,373). The implement is left in the ground after the post has been positioned. These types of post hole diggers are satisfactory for use in soft earth, but are difficult to use in hard or rocky soil. Augers may be stalled completely by rocks. In addition, applications such as park or highway maintenance require digging a single hole or a small number of holes at one location. Motorized equipment is impractical for on-the-spot maintenance and small-scale operations where only a few holes are needed, and water may not be available at all locations.
Manual post hole diggers are perhaps the most common type being sold today. The familiar "clamshell" digger consists of four main parts: two long pole-handles and two clamshell digging cups connected to the handles. The handles are moved together to separate the cups, and apart to bring the cups together. Digging is accomplished by plunging the digger into the ground with the handles together, followed by spreading the handles apart to force the cups together to grip a quantity of soil. The digger is pulled out, handles still held apart, and the load is then dumped off to the side. The process is repeated until the hole is the desired depth.
Any homeowner is familiar with this type of digger and its principal problem: as the hole is dug deeper, the handles cannot be spread apart sufficiently far to grip the soil unless the diameter of the top of the hole is made progressively larger--otherwise, the handles will bind with the top edge of the hole. This type of digger requires time and effort for the removal of additional soil, and provides a less firm foundation for a post set into the hole when the loosened soil is used as fill.
Many constant-hole-diameter post hole diggers have been developed in attempts to solve this problem. Some designs have scissors-type connections between the two poles to allow digging deeper holes before the hole needs to be enlarged (Stormsgaard, Des. No. 311,853; Weiland, U.S. Pat. No. 4,042,270). Most constant-hole-diameter diggers have a pair of opposing clamshell-type digging blades pivotally mounted at the lower end of a shaft. A pull rod inside the shaft opens and closes the blades. Such diggers are provided by Rice (U.S. Pat. Nos. 2,712,467; 2,654,626), Arens (U.S. Pat. No. 2,710,765), Sonnenberg (U.S. Pat. No. 2,435,473), Loos, et al. (U.S. Pat. No. 2,230,498), Mayeda, et al. (U.S. Pat. No. 2,028,680), McDowell (U.S. Pat. No. 1,888,929) and Theriot (U.S. Pat. No. 1,706,332). These devices are complex and expensive to make, and awkward to use. None of the devices can be extended in length for digging deeper holes.
There is a need for a constant-hole-diameter post hole digger that is simple and inexpensive to make, reliable, and simple to use.